The present invention relates generally to solid-phase adsorption/desorption techniques for recovering valuable chemical products. More particularly, the invention relates to highly efficient and economic processes for thermally desorbing adsorbed products, and apparatuses useful in conducting the processes.
As further background, the recovery and purification of carboxylic acids and other valuable chemical products from mediums has long been studied in an effort to discover efficient, cost-effective routes for their production. For example, carboxylic acids such as citric acid and lactic acid are manufactured by fermentation in large scale worldwide. Such fermentations provide fermentation broths from which the desired acid must be recovered and purified. Where high volume manufacture is involved, the importance of keeping recovery costs to a minimum cannot be overemphasized.
Recent recovery work has focused on the use of solid polymeric adsorbent materials to recover carboxylic acids from fermentation mediums. In this approach, the fermentation broth is passed over the adsorbent which adsorbs the carboxylic acid, and the carboxylic acid is desorbed in some fashion to provide product. Generally, a wide variety of adsorbents and adsorption/desorption schemes have been proposed.
For example, Kawabata et al., U.S. Pat. No. 4,323,702, describe a process for recovering carboxylic acids with a material of which the main component is a polymeric adsorbent having a pyridine skeletal structure and a cross-linked structure. The carboxylic acid is adsorbed on the adsorbent, and then desorbed using a polar organic material such as an aliphatic alcohol, ketone or ester. However, these polar organics can be difficult to separate from the eluted medium, and/or can cause significant side reaction(s) during operations such as distillation necessary for the separation.
Kulprathipanja et al., in U.S. Pat. Nos. 4,720,579, 4,851,573, 4,851,574, teach solid polymeric adsorbents including a neutral, noniogenic, macroreticular, water-insoluble cross-linked styrene-poly(vinyl)benzene, a cross-linked acrylic or styrene resin matrix having attached tertiary amine functional groups or pyridine functional groups, or a cross-linked acrylic or styrene resin matrix having attached aliphatic quaternary amine functional groups.
In their work, Kulprathipanja et al. describe "pulse tests" in which they identify acetone/water, sulfuric acid, and water as desorbents. Needless to say, an acetone/water desorbent leads to organic materials in the desorbed fraction and attendant disadvantages as discussed above. When sulfuric acid is used as desorbent, it of course is present in the eluted fraction and complicates product recovery. Moreover, although they name water as a potential desorbent, Kulprathipanja et al. indicate its unfeasibility in their processes, directing in their '573 patent that water "is not strong enough to recover the absorbed citric acid quickly enough to make the process commercially attractive."
South African Patent Application No. 855155, filed Jul. 9, 1985, describes processes in which product acids were recovered from their aqueous solutions. In the adsorption step, the acid-containing solution was passed through a column containing an adsorber resin consisting of a vinylimidazole/methylene-bis-acrylamide polymer, a vinylpyridine/trimethylolpropane tri-methacrylate/vinyltrimethylsilane polymer, a vinylimidazole/N-vinyl-N-methylacetamide/methylene-bis-acrylamide polymer, Amberlite IRA 35 (Rohm & Haas--acrylate/divinylbenzene based polymer containing dimethylamino groups), or Amberlite IRA 93 SP (Rohm & Haas) or Dowex MWA-1 or WGR-2 (Dow Chemical) (these latter three being styrene/divinylbenzene based polymers containing dimethylamino groups). To desorb the acid, water, usually at a temperature of 90.degree. C., was allowed to pass through the column. However, the single-pass elution process described involves an inefficient use of heat energy and does not substantially maximize the potential use of the resins to achieve highly concentrated desorbed solutions. Additionally, resins employed in this South African application are relatively thermally instable and thus substantially degrade during desorption procedures employing hot water.
International Applications PCT/US92/02107 filed Mar. 12, 1992 (published Oct. 1, 1992, WO 92/16534) and PCT/US92/01986 filed Mar. 12, 1992 (published Oct. 1, 1992, WO 92/16490) both by Reilly Industries, Inc., disclose desorbing lactic and citric acid, respectively, from divinylbenzene crosslinked vinylpyridine or other resins using steam or hot water. The resins employed have advantageous adsorption/desorption capacities and are highly thermally stable under the described hot water desorption procedures. Nonetheless, improved processes would provide greater efficiency in the use of heat applied to the desorption and would readily provide desorbed solutions of even higher product concentration.
In light of this and other background in the area, there remains a need for improved, effective processes for recovering carboxylic acids and other valuable products from their dilute solutions. The present invention addresses these needs.